Acoustic logging is one of the important measurement methods used to evaluate oil and gas reservoirs. For example, in rock mechanics analysis, acoustic data can provide pore pressure, rock strength, crustal stress orientation, and other information; and in petrophysical analysis, the acoustic data can be used to evaluate formation lithology and porosity. In recent years, an acoustic LWD technology is used to measure compressional wave and shear wave velocities while drilling, reduce the time taken by a well rig, improve the drilling efficiency, realize risk management in real time, and may meet needs of highly-deviated wells, horizontal wells and deep-water drilling. Accordingly, the acoustic LWD technology has been rapidly developed in oil and gas exploration and development.
An acoustic LWD instrument generally consists of an acoustic transmitting source, an acoustic receiver array and a acoustic insulator. When the instrument operates, the acoustic transmitting source periodically transmits acoustic pulses to the formation around the wellbore. An acoustic wave signal is received by the receiver array when the acoustic wave is propagating along the formation around the well, and the acoustic receiver array samples full-wave train signals to digitize the waveforms. The acoustic insulator is located between the acoustic transmitting source and the acoustic receiver array for attenuating direct wave signals propagating along a drill collar. The waveform data is processed, and a time difference of each component wave is calculated, the raw data is stored in a memory of a downhole instrument, and processing results of the time differences are transmitted to the ground through a mud pulse telemetry technology in real time. As a drilling continues, the above-mentioned acoustic signal measurement and data processing processes are repeated continuously, and the compressional wave and shear wave velocities of the formation at different depths are obtained.
The acoustic LWD technology has been undergoing the first generation of acoustic LWD instrument for measuring compressional waves and/or shear waves in fast formation by using a monopole source and the second generation of acoustic LWD instrument for measuring shear waves in slow formation by using a quadrupole source since it has been put into field in 1990s. However, these instruments may only provide single compressional and/or shear wave velocity information since a monopole measurement mode and a quadrupole measurement mode are both lack of azimuth characteristics, thereby being only suitable for logging applications in isotropic formations.
With an increasing demand for detailed evaluation of anisotropic complex reservoirs, the azimuthal acoustic LWD technology has received more and more attention in the industry in recent years. The technology usually adopts azimuthal acoustic source excitation, and utilizes rotation of the drill collar to obtain information about acoustic velocities in different directions in real time, for describing three-dimensional rock mechanical properties around the well.
The acoustic LWD instrument may have four operation modes according to different loading modes of the transmitting source: monopole, unipole, dipole and quadrupole, wherein the monopole mode and the quadrupole mode are difficulty applied in the azimuthal acoustic LWD instrument due to no azimuth characteristic or poorer azimuth characteristics caused by symmetrical loading.
The patent CN106930758A discloses an azimuthal acoustic LWD apparatus and method thereof, which adopts a quadrupole source transmission and multi-mode acquisition to determine anisotropic parameters of a formation by processing array signals. Since the quadrupole mode is insensitive to azimuth characteristics of the formation, the method may only determine a direction in which fast compressional waves are located and a size of an anisotropic value, and may not obtain information about velocity in different directions around the wellbore.
In addition, the above azimuthal acoustic LWD technology mainly involves a structure of a measurement apparatus and a data processing method, other than a azimuthal acoustic measurement method. The prior art generally adopts a fixed time interval measurement mode, that is, a transmitter transmits acoustic signals at fixed time intervals by programming, and a receiver records waveforms at the same time intervals, for example, measures them once per 100 ms. At the same time, a toolface angle of the instrument for each measurement is synchronously recorded (usually, an rotation angle of the instrument relative to a certain reference position), and measured data is assigned to different sectors according to the current value of the toolface angle, such an azimuthal coverage depends on the rotation speed and the measurement time interval of the instrument. When the measurement time interval has a specific relationship with the rotation speed of the instrument, for example, when the rotation speed is 1 revolution per second and the measurement time interval is 1 second, the instrument measures it only once in a fixed direction per revolution, may not cover positions around the entire wellbore, and may not achieve the azimuthal acoustic LWD. Accordingly, in view of deficiencies of the prior art, it is urgent to develop an effective azimuthal acoustic LWD apparatus and its associated measurement method.